One of the first olefinically unsaturated synthetic elastomers to be commercially produced was butyl rubber. The expression "butyl rubber" is used in the rubber industry to describe copolymers made from a polymerization reaction mixture having therein from 70 to 99.5% by weight of an isoolefin which has about 4 to 7 carbon atoms, e.g. isobutylene, and about 30 to 0.5% by weight of a conjugated multiolefin having from 4 to 14 carbon atoms, e.g. isoprene. The resulting copolymers contain 85 to 99.5% by weight combined isoolefin and about 0.5 to about 15% combined multiolefin.
The preparation of butyl rubber is described in U.S. Pat. No. 2,356,128, which is incorporated herein by reference. The polymer backbone of commercial butyl rubber is made up primarily of isobutylene units, with just a few percent isoprene units. The isoprene units contribute the small amount of unsaturation present in butyl rubber. The basic preparative equations are represented by: ##STR1## which combine to form its main structure: ##STR2## wherein n+1 represents the number of isoolefin units incorporated in the butyl rubber, while m represents the number of diolefin units present, substantially as isolated units. The conjugated diolefin loses its diene unsaturation upon its incorporation into the polymer backbone.
Thus butyl rubber, as presently produced, contains only a small percentage of unsaturation, in the form of the monoolefin structure associated with the isoprene residue which is incorporated more or less randomly throughout the polymer chain.
Numerous references teach methods for halogenating various polymers. Generally, these references are limited to reactions in solution or are batch rather than continuous processes. There are, for example, references that teach the halogenation of butyl rubber, but each suffers from serious limitations. An early reference, U.S. Pat. No. 2,944,578, teaches that chlorinated butyl rubber can be produced in a batch process by dissolving butyl rubber in a suitable, nonreactive solvent, e.g., hexane, and introducing chlorine or a chlorinating agent. By suitable control of the temperature, concentrations of chlorinating agent and rubber, and reaction time, chlorinated rubber containing the desired level of chlorine is produced. However, a batch process is inherently inefficient, and the need to dissolve the rubber in a solvent incurs significant expenses for solvent recovery and environmental control.
An improved, continuous process for chlorination or bromination of butyl rubber was subsequently disclosed in U.S. Pat. No. 3,099,644. However, that process still required the preparation and use of a solution of butyl rubber, which, in addition to the limitations noted above, is limited as to the concentration of rubber which can be processed, and which requires significant equipment and process control to precipitate the halogenated rubber from solution and then dry the rubber in a controlled manner so as to avoid degradation. The halogenation of ethylene-propylene nonconjugated diene elastomers (EPDM) has also been disclosed; such processes are analogous to those for halogenating butyl rubber. For example, U.S. Pat. No. 4,051,083 describes the solution bromination and chlorination of EPDM using N-halosuccinimide; additionally, the "neat" halogenation of EPDM is also described. In the latter disclosure the halogenating agent is dispersed in the EPDM by blending on a cool rubber mill and halogenation is effected by heating the mixture in a hydraulic press.
Halogenation of EPDM in an aqueous batch process is disclosed in U.S. Pat. No. 3,896,095. The process employs the addition of an excess of Cl.sub.2 or Br.sub.2 to a polymer slurry to effect halogenation and avoid the expense of solvent recovery systems previously disclosed for solution halogenation processes.
Chlorobromination of polymers such as polybutadiene, butadiene-isoprene copolymers and natural or synthetic polyisoprene is disclosed in British Pat. Nos. 1,483,063 and 1,483,064. The reaction is described as taking place at a low temperature of 0-15 degrees C., preferably in an inert solvent, and the halogenated products are described as containing high levels, e.g., at least 55% by weight of halogen.
A close reading of these references indicates the difficulty with which halogenation of elastomers has been conducted prior to the invention disclosed herein. The various limitations of these batch and continuous solution processes are overcome by the improved process of the present invention.
The possibility of producing a halogenated rubber such as halogenated butyl rubber continuously in an extruder-reactor has been recognized, see, e.g., U.S. Pat. No. 4,185,057. However, the generalized disclosures of that reference do no more than acknowledge the desirability of such a process, but do not teach one how to accomplish such a process. The reference suggests that only enough chlorine be introduced into the extruder to react with the butyl rubber so that no chlorine remains after reaction. It then goes on to suggest that another gas, e.g., nitrogen, be introduced to effect the production of gas filled pores in the finished rubber, which is the primary object of the invention. No examples are disclosed in the patent and no conditions disclosed which would enable one to actually conduct such a butyl halogenation process.
Chlorination of butyl rubber using dichloramine-T and a calendar has been reported by Bulgarian workers (Kh. Tenchev. et al., Chem Abstracts 50756u.) The disclosed process was not intended to produce neat chlorinated butyl since calendering is carried out on a mixture of butyl rubber, accelerators, prevulcanization inhibitors as well as variable amounts of carbon black and dichloramine-T.
The halogenation, in a kneader or extruder, of polymers containing carboxylic acid groups using reagents that differ from those disclosed herein is described in U.S. Pat. No. 3,364,187. The polymers are converted to the acyl halide derivatives using specific halogenating agents. The patent suggests that the kneading step may be carried out in an extruder, a Banbury mixer, a roll mill or any other apparatus that yields the described kneading action.
A British Pat. No. 1,257,016, discloses a process for treating polymers with halogenating agents such as N-bromosuccinimide under mechanical shear for the purpose of producing unsaturation. The patent mentions that halogenation may possibly occur in an intermediate step followed by dehydrohalogenation, but production and isolation of a useful halogenated product is not an objective, nor is it achieved. The process also requires the use of scavenging amounts of a metal oxide or carbonate such as magnesium oxide, zinc oxide or calcium carbonate in addition to the halogenating agent and -olefin polymer. The patent discloses, as an alternate method, the preblending of the halogenating agent with a solution of the polymer followed by solvent removal. It is stated that very little, if any, reaction occurs during such an operation.
An extensive disclosure of polymer modifications conducted in an extruder can be found in U.S. Pat. No. 3,862,265. This patent is directed to modification of polyolefins using heat, shear and controlled pressure to induce degradation in the polyolefin and to combine the polyolefin with a free-radical initiator and/or one or more monomers. The broad disclosure is of value for its teachings directed to the modification of polyolefins with various monomers especially to form novel grafted polymers.
Canadian Pat. No. 1,121,956 describes the treatment of blow-molded articles with fluorine gas to impart barrier properties to the article. It is achieved by introducing a mixture of fluorine and an inert gas into the interior surface of a parison before charging the parison into a blow-mold; the parison is then expanded by an inert gas under pressure. Such batchwise surface treatment method is not particularly relevant to the continuous whole-polymer modification process disclosed herein.
U.S. Pat. No. 3,510,416 (Vaccari et al) teaches an improved method of halogenating PVC particles by using gaseous hydrogen in combination with a swelling agent (chlorination carrier). Following reaction, the PVC particles are transferred to another piece of equipment (a dryer) in which the chlorination carrier is stripped and gaseous by-products are separated. This reference discloses a process based on particle fluidization which relies on diffusion to accomplish drying; in addition, such a process requires separate pieces of equipment and relatively long times for drying.
Some polymers are particularly sensitive when exposed to shear and elevated temperatures in the presence of a halogenating agent. For example, butyl rubber is subject to degradation under such conditions and this has made the achievement of a halogenated butyl product using an extruder-reactor a difficult goal, and, until the invention described at the end of this section, a goal that had not yet been achieved. The halogenation reaction of butyl rubber in solution is described in "Encyclopedia of Chemical Technology," Kirk-Othmer, Third Edition (1979), Volume 8 at page 476 ff. It is noted that the halogenation reaction carried beyond one halogen atom per olefin unit is complicated by chain fragmentation. Indeed, such fragmentation or degradation is a persistent problem when halogenation of butyl rubber is attempted; that problem is aggravated under conditions of heat and shear.
An additional difficulty in this field of polymer modification is the dehydrohalogenation reaction. One means of suppressing such a reaction is the addition of stabilizers which can be added, e.g., to a solution of halogenated butyl to protect against this reaction during processing. It is also necessary to avoid other undesirable side reactions which vary depending on the particular polymer being halogenated. Such reactions are further aspects of the sensitivity of the polymers to the severe halogenation reaction that has made the achievement of controlled halogenation of neat polymers in an extruder-reactor a previously elusive goal.
Other difficulties which are encountered in attempting to halogenate neat polymers include: the problem of mixing a highly viscous polymer phase with a low viscosity halogenating agent phase (e.g., where a gaseous halogenating agent is used this difference can be as much as ten orders of magnitude); the low probability of the halogenating agent encountering the reactive site on the polymer, particularly when a low functionality polymer is employed (e.g., butyl rubber, isobutylene/isoprene copolymer); and the difficulty of removing from contact with the polymer, i.e., disengaging, potentially damaging by-products of the reaction, e.g., hydrogen halide. These problems and others have been overcome by the invention disclosed herein.
The reactivity of the butyl rubbers and consequently their cure rate is substantially less than the high unsaturation natural and synthetic rubbers. In an effort to improve cure characteristics of the butyl rubbers, these synthetic polymers have been halogenated. Halogenated butyl rubber has contributed significantly to the elastomer industry. Some forms of halogenated butyl rubber, prepared in solution according to processes described above, are commercially available, e.g., chlorinated butyl rubber and brominated butyl rubber. One method used to prepare halogenated butyl rubber is that of halogenating butyl rubber in a solution (butyl rubber cement) containing between 1 to 60% by weight of butyl rubber in a substantially inert C.sub.5 -C.sub.8 hydrocarbon solvent such as pentane, hexane, heptane, etc., and contacting this butyl rubber cement with a halogen for a period of up to about 25 minutes. There is then formed the halogenated butyl rubber and a hydrogen halide, the polymer containing up to one or somewhat more halogen atoms per double bond initially present in the polymer. Generally, halogenated butyl rubber comprises a copolymer of 85 to 99.5% wt.% of a C.sub.4 to C.sub.8 isoolefin, e.g., isobutylene, with 15 to 0.5 wt.% of a CH.sub.4 to C.sub.14 multiolefin, e.g., isoprene, containing at least about 0.5 wt.% combined halogen in its structure. For example, where butyl is halogenated with bromine, the bromine can be present in the brominated butyl in an amount of from about 1.0 to about 3.0 wt.%, preferably from about 1.5 to about 2.5 wt.%. A method of preparing conventionally halogenated butyl rubber is described in U.S. Pat. No. 3,099,644, referred to above, which is incorporated herein by reference.
The preparation, in solution, of halogenated butyl rubber containing both bromine and chlorine, i.e., bromochlorinated butyl rubber, is described in U.S. Pat. No. 4,254,240, incorporated herein by reference. The potential for molecular weight breakdown of the butyl rubber, noted earlier, is present even where bromine chloride is used as the halogenating agent, as disclosed in this reference (column 4, lines 24-32). The structural formula for halogenated butyl rubber is typically represented as being: ##STR3## where X represents the halogen and, 1 and m have the same values as described above for butyl rubber. This structure, however, is one of several which can be formed, depending on the conditions of halogenation, the halogenating agent, used etc. Other structural configurations which may occur in halogenated butyl rubbers are ##STR4## It will be noted that in each case the halogen is present as a secondary or tertiary allylic halogen.
More recently, U.S. Pat. No. 4,288,575 to Irwin Gardner (which has an effective filing date of Mar. 7, 1977) discloses a new structural configuration for the halogenated rubber where the rubber contains conjugated diene which is represented as ##STR5##
In this structure the halogen, X, is in a primary allylic position. The method disclosed in U.S. Pat. No. 4,288,575 for preparing these rubbers involves the use of a copper oxide catalyst useful for dehydrohalogenation of butyl rubber to form a conjugated diene rubber.
As shown in Example 6 of the Gardner '575 patent this primary halogen is in a more stable configuration than the secondary halogens of the prior art and is not readily removed. The copper oxide catalyst was taught in Gardner's earlier U.S. Pat. No. 4,145,492 to be a dehydrohalogenation catalyst suitable for the preparation of conjugated diene rubber.
Table I of U.S. Pat. No. 4,288,575 shows various halogenated conjugated diene-containing polymers which are shown to have the halogen in the primary position. Not surprisingly, the residual halogen is always associated with substantial amounts of conjugated diene.
Where high amounts of residual halogen are present in the polymer, as in Run A of Table I of U.S. Pat. No. 4,288,575, it is the result of an initially high level of halogenation; here 1.95 wt. % bromine. Since the degree of rearrangement is proportional to the degree of dehydrohalogenation Gardner's polymers cannot be low in conjugated diene and at the same time have appreciable amounts of halogen present in the primary allylic position.
In 1979 Van Tongerloo et. al. disclosed a brominated butyl rubber which was low in conjugated diene content (if any) and had the primary halogen configuration. The polymer is represented as having the structure. ##STR6## The reference states that the polymer was produced by a proprietary method and Van Tongerloo et. al. disclose only that rearrangement to the more stable primary configuration can be accomplished in brominated butyl rubber "under a variety of conditions--for example, in the presence of acid, free radicals, bases or heat." See Van Tongerloo, A. and Vokov, R., Proceedings, International Rubber Conference, Milan, Italy, 1979, p. 70ff. The skilled chemist will recognize that this gratuitous disclosure represents the techniques which can be enumerated to accomplish an infinite number of reactions. The disclosure in no way teaches any method to prepare the polymer disclosed.
Van Tongerloo et. al. designate the methylene configuration of Formula VI above as "EXO" and the primary bromo configuration of Formula V as "ENDO." It is alleged that even at ratios of ENDO: EXO of 71:16 there is no clear indication of a correlation between vulcanizate properties and polymer microstructure. Hence, Van Tongerloo et. al. have not appreciated that the polymer which they purportedly made by an undisclosed proprietary process has any properties which are different than those of conventional halogenated butyl rubber.
Subsequent to the making of the instant invention, Vukov disclosed that certain model compounds can be heated to 150 degrees C. for 30 minutes to accomplish a molecular rearrangement as follows: ##STR7## No substantial rearrangement of the chlorinated model was observed. See Vukov, R., "Halogenation of Butyl Rubber and The Zinc Oxide Cross-Linking Chemistry of Halogenated Derivatives" which was presented to the ACS Rubber Division on Oct. 25, 1983. Those skilled in the art will recognize that what is true about simple molecules (model compounds) may not necessarily be true about complex polymer molecules.
Conventional processes, which halogenate polymers such as butyl rubber in solution, incur significant disadvantages. These include high capital investment for the equipment needed to handle, purify, and recycle the solvent, high energy costs for the movement, vaporization, and purification and recycle of the solvent, potential halogenation of the solvent, potential hydrocarbon emissions to the atmosphere and the use of considerable space for the equipment necessary to handle large volumes of solutions.
A previous patent application, filed by two of the inventors herein (U.S. Ser. No. 306,882), filed Sept. 30, 1981 now U.S. Pat. No. 4,384,072 issued May 17, 1983) disclosed an improved halogenation process in which neat rubber was halogenated in an extruder. A significant feature of the earlier invention was injection of the halogenating agent at a position filled with rubber and subjecting the rubber and agent to a high degree of mixing. The invention disclosed herein is a further, significant improvement over such a process.